Comment # 21 on bug 916781 from Johannes Meixner

In your error_log_1print.txt in attachment#622879 [details]
there is:
-------------------------------------------------------------------------
$ grep PID error_log_1print.txt

I [11/Feb/2015:17:45:10 +0000] [Job 9]
 Started filter /usr/lib/cups/filter/pdftops (PID 3304)
I [11/Feb/2015:17:45:10 +0000] [Job 9]
 Started backend /usr/lib/cups/backend/parallel (PID 3305)
D [11/Feb/2015:17:45:10 +0000] [Job 9]
 Started filter pdftops (PID 3306)
D [11/Feb/2015:17:45:10 +0000] [Job 9]
 Started filter pstops (PID 3307)
D [11/Feb/2015:17:45:11 +0000] [Job 9]
 PID 3306 (pdftops) exited with no errors.
-------------------------------------------------------------------------

Only one preocess finished.
The other ones, in particular /usr/lib/cups/backend/parallel
seem to just hang because there are no error messages.

I guess /usr/lib/cups/backend/parallel endlessly hangs waiting
to connect to the printer via the parallel port but that does somehow
not work.

I guess while /usr/lib/cups/backend/parallel endlessly hangs
there is "/dev/lp0: Device or resource busy".

Anthony Lee,
ar root run

cancel -a

to stop and remove all pending/active print jobs,
then reboot your system and retry the various
  echo -en "\rHello\r\f" >/dev/lp0
tests to directly access your printer.